CI: tweak concurrency limiting

[codecov[bot]]

Codecov Report

Merging #25 (671a2ff) into master (eb1dc3a) will decrease coverage by 0.95%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
- Coverage   73.95%   73.00%   -0.96%     
==========================================
  Files         343      342       -1     
  Lines         576      563      -13     
==========================================
- Hits          426      411      -15     
- Misses        150      152       +2     

Impacted Files	Coverage Δ
lib/parunits.g	0.00% <0.00%> (-5.00%)	:arrow_down:
lib/unitlib.g	80.68% <0.00%> (-0.43%)	:arrow_down:
lib/unitlib.gd